A. Field
The present invention concerns a fluid-injected screw-type compressor element comprising two co-operating rotors which are bearing-mounted in a housing, whereby this housing confines a rotor chamber in which the rotors are situated and in which a fluid circuit for the injection of a fluid discharges.
B. Related Art
Fluid-injected screw-type compressor elements are generally known and work according to the known principle whereby the volume of the compression chambers which are formed between the teeth of the respective rotors meshing into one another gradually decreases while the rotors are turning.
After a certain rotation of the rotors, one of the compression chambers which is formed between the rotors is connected to the outlet of the screw-type compressor element.
Oil-injected screw-type compressor elements are already known at present whereby, for the lubrication, cooling and sealing of the rotors, oil with a relatively high kinematic viscosity of 30 to 70 cSt (=3·10−6 to 7·10−6 m2/s) at 40° C. according to ISO 46 is injected in the compression chamber.
The good working of such oil-injected screw-type compressor elements requires the use of radial and/or radial/axial roller bearings on either side of both rotors, so that said rotors can be mounted in a rotating manner in such a way that the friction between the rotors and the housing is restricted.
By roller bearings, or what are called anti-friction bearings, are understood bearings which are provided with roller elements which can be formed in various ways, such as in the shape of balls, tapered rollers, cones, cylinders, needles or the like.
The presence of such radial or radial/axial roller bearings is required among others to absorb radial forces exerted on the rotors during the compression, as well as the forces which are exerted on said rotors by the drive, for example in the form of gear wheels, belts or chains. Said roller bearings also make sure that the transmission of vibrations from the rotors to the housing is limited, which is good for the life span of the compressor and also to restrict any noise nuisance.
A disadvantage of such known screw-type compressor elements is that such roller bearings are usually expensive and relatively difficult to mount during the assembly of the screw-type compressor element.
Water-injected compressor elements are already known as well whereby, instead of oil, water is injected in the compression chamber with the same purpose, i.e. to provide for a cooling, lubrication and sealing.
With this type of water-injected screw-type compressor elements, the rotors are radially bearing-mounted with axle journals on either side in the housing by means of water-lubricated slide bearings comprising a corrosion-free bush, for example made of graphite, which bush extends between the respective axle journals of the rotors on the one hand, and the housing of the compressor element on the other hand.
A disadvantage of such water-injected compressor elements is that such water-lubricated slide bearings are usually expensive as the aforesaid bush is always made of a relatively expensive material and as said bearings require a high manufacturing tolerance of the bush.
Another disadvantage of such water-injected screw-type compressor elements is that they often have a relatively complex housing due to the presence of several injection channels for injecting water in the compression chamber and in the slide bearings.
An additional disadvantage of the known water-injected screw-type compressor elements is that the play of the rotors in the housing is relatively large, as a result of which leakage losses occur in the compression chamber between the mutual rotors and between the rotors and the wall of the rotor chamber, which leakage losses are partly absorbed by the injected fluid.
The present invention aims to remedy one or several of the above-mentioned and other disadvantages.